SUMMARY

RFC 6186 (https://www.rfc-editor.org/rfc/rfc6186) describes a way for MUAs to locate the correct submission, IMAP and/or POP3 servers for certain mail addresses. In short, it works as follows:
1. ask the user for its email address & password
2. extract the domain part of that address
3. request DNS SRV records for _{submission,imap,imaps,pop3,pop3s}._tcp.<domain>.
4. (if both IMAP & POP3 can be used & MUA supports both) decide between IMAP and POP3 according to SRV priority, or preference of MUA
5. first try <localpart>@<domain> as username, then only <localpart> as fallback
6. profit

IMO RFC 6186 is simple to implement for MUAs as well as mail administrators and it is sufficient for most email server setups. Comparing to Microsoft’s autodiscover & Thunderbird’s autoconfig variants (which do support more weirder setups), RFC 6186 is only based on DNS entries, so it is not required for mail administrators to create rather complex XML files and serve them via HTTP(s).

SECURITY

Some recommend the use of DNSSEC for validating DNS SRV records. If the targets of the SRV records points to subdomains on the same "main domain", such validation might be lifted. Here by "main domain" I mean domains which can be registered arbitrarily. For validating if two domains correspond to the same "main domain" the public suffix list can be used, see https://publicsuffix.org/.

RELEVANCE

RFC 6186 sadly is not (yet) supported by major email providers nor by major email clients.

The following public mail providers support RFC 6186 as of now (2024-02-24):
- icloud.com
- yandex.com

Support by other MUAs:
- Microsoft Outlook (Desktop): unknown
- Mozilla Thunderbird: not yet, see https://bugzilla.mozilla.org/show_bug.cgi?id=342242
- K-9 Mail: planned, see https://github.com/thunderbird/thunderbird-android/issues/4721#issuecomment-1732575441